Long-Term Evolution (LTE) systems offer high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. An LTE system also provides seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). Enhancements to LTE systems are considered so that they can meet or exceed IMA-Advanced fourth generation (4G) standard. One of the key enhancements is to support bandwidth up to 100 MHz and be backwards compatible with the existing wireless network system. In LTE/LTE-A systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) communicating with a plurality of mobile stations, referred as user equipments (UEs).
Typically, each UE needs to periodically measure the received signal quality of the serving cell and neighbor cells and reports the measurement result to its serving eNB for potential handover or cell reselection. The measurements may drain the UE battery power. In order to keep UE battery consumption low, the UE needs to toggle between sleeping and awake states. Preferably it should be possible for UEs in connected mode to apply similar sleep/awake performance as in Idle mode, to have similar battery consumption as in Idle mode. To save power, Discontinuous Reception (DRX) needs to be used in Connected mode, with short awake times and long sleep cycles. With DRX extension, UEs are configured with longer Connected mode DRX cycle.
Despite the benefit of power saving, one major drawback of DRX extension is the handover performance degradation. The performance of the current network-controlled handover procedure, which is based on signaling in both source cell and target cell, is dependent on triggering the handover procedure at the best moment in time, which in turn depends on factors such as UE speed, radio deployment, and DRX cycle. When DRX is applied, radio resource management (RRM) measurement is performed only within DRX ON durations, and longer DRX cycle leads to sparser measurement. In the most common failure case, the handover trigger is too late, and the radio link quality degrades below minimum requirement for successful transmission before handover complete, resulting in handover failure (HoF) or radio link failure (RLF). In essence, the UE has already moved outside sufficient radio coverage of its serving cell when the UE wakes up to perform a measurement and delivers a measurement report to the eNB of the serving cell, leading to failure either in the sending/receiving the measurement report from the UE to the eNB, or in the sending/receiving of subsequent reconfiguration or handover command message from the eNB. Thus, a high connection failure rate would be a normal case for a UE applying extended DRX cycle.
When a connection failure happens, in current LTE systems, several signaling messages are required to recover the radio connection. First, a signaling radio bearer (SRB), then in subsequent step additional SRBs and data radio bearers (DRB), are needed to transport application user data, and to restart DRX operation allowing the UE to go to sleep. Therefore, the increase of connection failures due to DRX extension in the current system reduces the usefulness of long DRX cycle, and puts limits to the battery autonomy.
A solution is sought to improve the mobility performance for UEs configured with extended connected mode DRX cycle in LTE systems.